The body has 2 external surfaces - the skin and several mucosal surfaces such as the mouth, lining of the gut, nasal passages, airways, urinary tract and genitals, notably the vagina in women. Of these the mouth, oesophagus (gullet) and vagina are relatively frequently infected by fungi. Only one fungus is exquisitely adapted to cause infection at these three sites – Candida albicans.

Thrush is the general term used for mucosal infection by Candida in the mouth and vagina. Candida grows on the surface of the mucosa, with minimal penetration, beyond the surface layer of cells. In some people there is virtually no inflammatory response and there is a lot of Candida, leading to white ‘plaques’ on the surface of the mucosa (pseudomembranous) or considerable vaginal discharge. In other people, there is marked inflammatory response leading to a reddened (erythematous) mucosa and considerable irritation and discomfort. In the mouth dentures may be coated with Candida which causes a local inflammatory reaction, called denture stomatitis.

Thrush is often recurrent because Candida albicans is a commensal organism, mostly living in harmony with humans in the gut and on mucosal surfaces in very low numbers. Only when that balance is disrupted does thrush occur.